1. Field of the Invention
The present invention relates to a light-emitting diode (LED) driver circuit. More particularly, the present invention relates to a buck converter LED driver circuit.
2. Description of the Related Art
An LED is similar to a silicon p-n junction diode. At its operating range, a slight change of forward voltage results in a large change in its operating current. Therefore, an LED requires constant current drive, not constant voltage drive. Any surge current above its rated current value will tend to degrade or even damage the LED.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram showing a conventional LED driver circuit with a buck converter topology. FIG. 2 shows some important waveforms in the LED driver circuit in FIG. 1. The alternating current (AC) voltage source 101 drives LEDs 103 through bridge rectifier 102. LEDs 103, inductor 104, and diode 105 are coupled as a loop. Here diode 105 is a fast-switching free-wheeling diode. Clock generator 106 provides a clock signal to the setting end (S) of SR flip-flop 108 so that the setting end is triggered and power switch Qm is turned on at each clock pulse. As power switch Qm is turned on, the current through LEDs 103 and inductor 104 gradually increases. At this time diode 105 is biased backward and does not conduct. Therefore the current through resistor Rsen is equal to the current through LEDs 103. When the LED current increases to the point where the voltage across resistor Rsen is higher than 0.5V, comparator 107 triggers the resetting end (R) of SR flip-flip 108 and power switch Qm is turned off. As power switch Qm is turned off, the LED current circulates in the loop formed by LEDs 103, inductor 104 and diode 105, decreasing gradually due to energy dissipation of LEDs 103 until the next clock pulse. As a result, the LED current exhibits a periodic zigzag waveform with a substantially constant level as shown in FIG. 2.
To assure the LED current is continuous, a large capacitor Cin, is connected between the bridge rectifier and the buck converter to hold up the input DC voltage Vcin such that Vcin is always higher than Vf, which is the voltage across LEDs 103. Without capacitor Cin, as the rectified input voltage Vin falls below Vf, the LED current would cease to flow. Therefore, the conventional driver circuit in FIG. 1 requires a large capacitor Cin and the input current Iin exists only when the rectified input voltage Vin is higher than the input DC voltage Vcin, as shown in FIG. 2. The large capacitance of Cin leads to a narrow range of conducting phase angle and a very poor input power factor. As shown in FIG. 2, the input current Iin conducts only for a small portion of the AC cycle time. The power factor is typically less than 0.65.
For a conventional buck converter LED driver circuit to feature a higher power factor, a solution is to incorporate a power factor correction (PFC) front-end as shown in FIG. 3. FIG. 3 is a schematic diagram showing a conventional buck converter LED driver circuit with a boost PFC front-end controlled by a PFC boost control circuit 110. Although the driver circuit in FIG. 3 has a higher power factor, it is far more complex than the driver circuit in FIG. 1. In many LED lamp fixtures, there is not sufficient space for the additional components.